CodeIgniter lets you build error reporting into your applications using
the functions described below. In addition, it has an error logging
class that permits error and debugging messages to be saved as text
files.

Note

By default, CodeIgniter displays all PHP errors. You might
wish to change this behavior once your development is complete. You’ll
find the error_reporting() function located at the top of your main
index.php file. Disabling error reporting will NOT prevent log files
from being written if there are errors.

Unlike most systems in CodeIgniter, the error functions are simple
procedural interfaces that are available globally throughout the
application. This approach permits error messages to get triggered
without having to worry about class/function scoping.

CodeIgniter also returns a status code whenever a portion of the core
calls exit(). This exit status code is separate from the HTTP status
code, and serves as a notice to other processes that may be watching of
whether the script completed successfully, or if not, what kind of
problem it encountered that caused it to abort. These values are
defined in application/config/constants.php. While exit status codes
are most useful in CLI settings, returning the proper code helps server
software keep track of your scripts and the health of your application.

This function will display the error message supplied to it using
the error template appropriate to your execution:

application/views/errors/html/error_general.php

or:

application/views/errors/cli/error_general.php

The optional parameter $status_code determines what HTTP status
code should be sent with the error. If $status_code is less
than 100, the HTTP status code will be set to 500, and the exit
status code will be set to $status_code+EXIT__AUTO_MIN.
If that value is larger than EXIT__AUTO_MAX, or if
$status_code is 100 or higher, the exit status code will be set
to EXIT_ERROR.
You can check in application/config/constants.php for more detail.

This function will display the 404 error message supplied to it
using the error template appropriate to your execution:

application/views/errors/html/error_404.php

or:

application/views/errors/cli/error_404.php

The function expects the string passed to it to be the file path to
the page that isn’t found. The exit status code will be set to
EXIT_UNKNOWN_FILE.
Note that CodeIgniter automatically shows 404 messages if
controllers are not found.

CodeIgniter automatically logs any show_404() calls. Setting the
optional second parameter to FALSE will skip logging.

This function lets you write messages to your log files. You must
supply one of three “levels” in the first parameter, indicating what
type of message it is (debug, error, info), with the message itself
in the second parameter.

Example:

if($some_var==''){log_message('error','Some variable did not contain a value.');}else{log_message('debug','Some variable was correctly set');}log_message('info','The purpose of some variable is to provide some value.');

There are three message types:

Error Messages. These are actual errors, such as PHP errors or
user errors.

Debug Messages. These are messages that assist in debugging. For
example, if a class has been initialized, you could log this as
debugging info.

In order for the log file to actually be written, the
logs/ directory must be writable. In addition, you must
set the “threshold” for logging in
application/config/config.php. You might, for example,
only want error messages to be logged, and not the other
two types. If you set it to zero logging will be disabled.